1. Field of the Invention
The present invention relates to a novel ferroelectric liquid crystal, a ferroelectric chiral smectic liquid crystal mixture, and a liquid crystal device constituting such a liquid crystal or liquid crystal mixture. More particularly, the present invention relates to a novel ferroelectric liquid crystal mixture having an improved response speed, and a liquid crystal device constituting such mixture which can be used as an optical shutter or a display device.
Further, the present invention relates to a ferroelectric liquid crystal for a .tau.-Vmin mode having a wide driving temperature margin, and a liquid crystal device comprising the same.
2. Description of the Related Art
With the recent progress in the exchange of information in society, importance of a display device as one of interfaces between man and machine has been greatly increased. Among such display devices, a flat panel display device such as a liquid crystal display (LCD) has quickly spread since it has various advantages such as a thin thickness, a light weight, a low driving voltage, a low power demand, and the like. Among the liquid crystal devices represented by the liquid crystal display, a matrix-addressed liquid crystal device having a large capacity of information includes two types of driving systems, namely an active-matrix-addressed system and a passive-matrix-addressed system.
In the active-matrix-addressed system, a thin film transistor or diode made of polysilicon or amorphous silicon is connected to each picture element as a non-linear element. However, the active-matrix-addressed system may have some problems in assembling a large area display, lowering the production cost or increasing density because of complicated production steps and low yield. In view of the cost and productivity, the passive-matrix-addressed system is advantageous.
As liquid crystal devices of the passive-matrix-addressed LC display which are currently practically used, TN liquid crystal and STN liquid crystal devices are mainly used. However, their optical response utilizes an average orientation of a molecule axis of the liquid crystal in a specific direction based on anisotropy of a dielectric constant of the liquid crystal molecule, which is induced by the application of an electric field. The, a limit of an optical response speed of such devices is on the order of a millisecond, and such response speed is insufficient in view of the increase of the amount of information. When the number of scanning lines is increased to increase the capacity of information, the decrease of the contrast ratio or cross-talk cannot be avoided inevitably. Those are the essential problems since the TN or STN liquid crystal device has no memory property (bistability). To solve such problems, various driving methods such as a dual frequency driving method, a voltage averaging method, a multimatrix method, and the like have been proposed. But, they cannot provide the fundamental solution of the problems. By such methods, it is difficult to increase the capacity or the density. Further, the TN or STN liquid crystal device has serious problems such as limitation of a viewing angle or quality of display.
To solve the essential problem of the above liquid crystal devices, in 1980, N. A. Clark and S. T. Lagerwall proposed a liquid crystal element utilizing a liquid crystal having bistability (see U.S. Pat. No. 4,367,924 and Japanese Patent KOKAI Publication No. 107216/1981). As the liquid crystal having the bistability, there is used a ferroelectric liquid crystal which has a chiral smectic C phase.
One of the advantages achieved by the use of the ferroelectric liquid crystal is that it has. Bistability is a property such that, when a ferroelectric liquid crystal is held between a pair of glass plates each carrying a transparent electrode, the ferroelectric liquid crystal has two different optically stable states depending on directions of the applied electric field, and the two optically stable states are maintained after the removal of the electric fields. Because of such a property, a liquid crystal device utilizing the ferroelectric liquid crystal is expected not to suffer from the decrease of a contrast ratio or cross-talk even when the number of scanning lines is increased.
Another characteristic of the ferroelectric liquid crystal resides in a high response speed. That is, the optical response of the ferroelectric liquid crystal is about 1000 times faster than that of the TN or STN liquid crystal, since the former utilizes a change of the orientation of the liquid crystal molecules caused by the direct interaction between spontaneous polarization of the ferroelectric liquid crystal and the electric field.
Accordingly, the ferroelectric liquid crystal has the following essential characteristics:
(1) It has two optically stable states, and those optically stable states are maintained after the removal of the electric field (bistability), and PA1 (2) The above two optically stable states can be switched on an order of a microsecond (high response speed). PA1 at least one compound of the formula (I): ##STR3## PA1 at least one compound of the formula (II): ##STR5## PA1 at least one compound of the formula (I), PA1 at least one compound of the formula (II), and PA1 at least one compound selected from the group consisting of a compound of the formula (V): ##STR7## PA1 at least one compound of the formula (I), PA1 at least one compound of the formula (III): ##STR13## PA1 at least one compound of the formula (IV): ##STR15##
In addition, in the ferroelectric liquid crystal device, the liquid crystal molecules respond to the electric field in parallel with the substrate and a cell thickness is made thin, so that dependency of the display on the viewing angle is small (large viewing angle).
Consequently, the ferroelectric liquid crystal device does not require the expensive non-linear element as in the case of the active-matrix-addressed system, and is expected to provide a high quality large display which can achieve the large information capacity and high quality display of the passive-matrix-addressed system.
Recently, Matthew Francis Born proposes an addressing method of a matrix-array liquid crystal cell comprising a liquid crystal material which is adjusted to show the minimum response time at a specific voltage (.tau.-V.sub.min mode) (Japanese Patent KOKAI Publication No. 20715/1991). In this addressing method, the liquid crystal device is driven by a positive slope portion of a dependency of the response time on the voltage using the property of the liquid crystal that it shows the minimum response time at the specific voltage. When such a driving method is used, it is expected that a good quality image without flickering can be set up.
A liquid crystal material which is suitable for the .tau.-V.sub.min mode should have a negative anisotropy of a dielectric constant at least in a frequency range between 1 kHz and 40 kHz in addition to properties which are required for the ferroelectric liquid crystal materials such as a phase sequence necessary for achieving a good orientation state, that is, a phase sequence in which, on slow cooling, an isotropic phase is changed to a chiral smectic C phase through a cholesteric phase and then a smectic A phase, a low viscosity and a high response speed.
Hitherto, several liquid crystal materials for the .tau.-V.sub.min mode are reported in scientific literatures or presentations in scientific meetings. However, there are still some problems to be solved. One of such problems is to develop a liquid crystal material which has a chiral smectic C phase in a sufficiently wide temperature range and a minimum response at a specific voltage. For example, mixtures described in The Prepublished Texts for The 19th Liquid Crystal Conference (1993), pages 249 and 251 are excellent as liquid crystal materials for the .tau.-V.sub.min mode. However, the highest temperatures of the chiral smectic C thereof are not satisfactory from the practical view point.
Then, hitherto, a large number of researches on the liquid crystal materials having the ferroelectricity have been made and reported. Before putting a ferroelectric liquid crystal device to practical use, still some problems are present. In particular, it is desired to provide a liquid crystal which has a high response speed and a small temperature dependency of properties relating to driving conditions.